Method for operating storage device determining wordlines for writing user data depending on reuse period

ABSTRACT

Embodiments include a method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining a set of wordlines of the selected memory block for writing data based on the reuse period of the selected memory block; and writing the data into the set of wordlines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This US non-provisional patent application claims priority under 35 USC §119 to Korean Patent Application No. 10-2015-0118995, filed on Aug. 24, 2015, the entirety of which is hereby incorporated by reference.

BACKGROUND

Embodiments relate to semiconductor memories and, more particularly, to methods for operating a storage device including a flash memory.

A storage device is a device that stores data according to the control of a host device such as a computer, a smartphone, and a smart pad. Storage devices include a device that stores data on a magnetic disk such as a hard disk drive (HDD) and a device that stores data in a semiconductor memory, particularly a nonvolatile memory such as a solid state drive (SSD) or a memory card.

Nonvolatile memories include a read-only memory (ROM), a programmable ROM (PROM), an electrically programmable ROM (EPROM), an electrically erasable and programmable ROM (EEPROM), a flash memory, a phase-change RAM (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), a ferroelectric RAM (FRAM), and the like.

With the advance in semiconductor manufacturing technology, highly integrated and high capacity storages device are being developed. The high integration of a storage device causes the manufacturing cost to be reduced. However, as a storage device has been scaled down due to the higher integration of the storage device, various problems have arisen. Due to the new problems, data stored in a storage device may be damaged, degrading reliability of the storage device.

SUMMARY

Embodiments include a method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining a set of wordlines of the selected memory block for writing data based on the reuse period of the selected memory block; and writing the data into the set of wordlines.

Embodiments include a method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining types of programming to use to program wordlines of the selected memory block based on the reuse period; and programming the wordlines using the types of programming.

Embodiments include a method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining a programming technique to use to program the selected memory block based on the reuse period; and programming the selected memory block using the programming technique.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing and other features will be described below in more detail with reference to the accompanying drawings of non-limiting embodiments in which like reference characters refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular embodiments. In the drawings:

FIG. 1 is a block diagram of a user device according to some embodiments;

FIG. 2 is a block diagram of an example of a flash memory in FIG. 1;

FIG. 3 is a circuit diagram of an example of a memory block BLK1 in FIG. 2;

FIG. 4 is a block diagram of an example of a memory controller in FIG. 1;

FIG. 5 is a flowchart summarizing an example of an operation of a time calculator in FIG. 4;

FIG. 6 is a flowchart summarizing an example of an operation where the user device in FIG. 1 selects a memory block to be erased;

FIG. 7 is a conceptual diagram illustrating an example of the number of erases (NOE) and the number of fast cycle (NOF) of the memory block described in FIG. 6;

FIG. 8 is a block diagram illustrating an example of an occurrence of fast cycles (FC) in FIG. 7;

FIG. 9 is a flowchart summarizing an example of a method for counting the number of fast cycles (NOF) in FIG. 7;

FIGS. 10A and 10B are flowcharts summarizing examples of S350 in FIG. 9;

FIG. 11 is a flowchart summarizing an example of a method in which the storage device in FIG. 1 dynamically controls wordlines of a memory block depending on a reuse period;

FIG. 12 illustrates an example of an operation of determining wordlines for writing user data depending on the reuse period described in FIG. 11;

FIG. 13 is a flowchart summarizing an example of a method for determining which one of a current reuse period (current R/P) and a previous reuse period (previous RIP) is used when dummy skip is executed depending on the reuse period described in FIG. 11;

FIG. 14 is a flowchart summarizing an example of a method in which the storage device in FIG. 1 adjusts a voltage provided to a low-reliability wordline depending on a reuse period;

FIG. 15 illustrates an example of an operation of determining wordlines for a normal program operation depending on the reuse period described in FIG. 14;

FIG. 16 is a timing diagram illustrating an example of a normal program voltage for a normal program operation and a slow program voltage for a slow program operation in FIG. 15;

FIG. 17 illustrates applying a storage device according to some embodiments to a memory card;

FIG. 18 is a block diagram illustrating an example of applying a storage device according to some embodiments to a solid state drive (SSD);

FIG. 19 is a block diagram illustrating an example of the configuration of an SSD controller in FIG. 18; and

FIG. 20 is a block diagram illustrating an example where a storage device according to some embodiments is implemented with an electronic device.

DETAILED DESCRIPTION

Embodiments will now be described more fully with reference to the accompanying drawings, in which particular embodiments are shown. Embodiments may, however, take many different forms and should not be construed as being limited to the particular embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those of ordinary skill in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference characters and/or numerals in the drawings denote like elements, and thus their description may be omitted.

However, the methods and systems will operate effectively in other implementations. Phrases such as “exemplary embodiment”, “one embodiment” and “another embodiment” may refer to the same or different embodiments as well as to multiple embodiments. The embodiments will be described with respect to systems and/or devices having certain components. However, the systems and/or devices may include more or less components than those shown, and variations in the arrangement and type of the components may be made without departing from the scope of the embodiments. The embodiments will also be described in the context of particular methods having certain steps. However, the methods may include different and/or additional steps and steps in different orders that are not inconsistent with the embodiments. Thus, embodiments are not intended to be limited to the particular embodiments shown, but are to be accorded the widest scope consistent with the principles and features described herein.

It will be understood by those skilled in the art that, in general, terms used herein, and especially in the appended claims are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to examples containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. Furthermore, in those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

FIG. 1 is a block diagram of a user device 1000 according to some embodiments. As illustrated, the user device 1000 includes a storage device 1100 and a host 1200. The storage device 1100 may be electrically connected to the host 1200 and configured to communicate with the host 1200.

The storage device 1100 may include a solid state drive (SSD) and memory cards such as a personal computer memory card international association (PCMCIA), a compact flash card (CF), smart media cards (SM and SMC), a memory stick, multimedia cards (MMC, RS-MMC, and MMCmicro), SD cards (SD, miniSD, microSD, and SDHC), a universal serial bus (USB) memory card, and a universal flash storage (UFS). The storage device 1100 may include an embedded memory such as embedded multimedia card (eMMC), UFS, and perfect page new (PPN). Although particular examples have been given, the storage device 1100 may take other forms.

The memory controller 1120 includes a random access memory (RAM) 1123 and a time calculator 1128. In FIG. 1, the RAM 1123 may be included inside the memory controller 1120. However, in other embodiments, the RAM 1123 may be disposed outside the memory controller 1120.

The memory controller 1120 may be configured to control the overall operations of the flash memory 1110, such as read or write operations, in response to a request of the host 1200. For example, the memory controller 1120 may be configured to receive data from the host 1200 and store the received data in the RAM 1123. The memory controller 1120 may be configured to write the data stored in the RAM 1123 into the flash memory 1110.

The memory controller 1120 may include a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit, a microcontroller, a programmable logic device, discrete circuits, a combination of such devices, or the like. The processor 102 may include internal portions, such as registers, cache memory, processing cores, or the like, and may also include external interfaces, such as address and data bus interfaces, interrupt interfaces, or the like. The memory controller 1120 may be configured to use these processing circuits and interfaces to implement the operations described herein.

The RAM 1123 may be used as a buffer memory, a cache memory, and/or a working memory. The RAM 1123 may be implemented with various types of memories such as dynamic RAM (DRAM), static RAM (SRAM), synchronous DRAM (SDRAM), phase-change RAM (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM), and ferroelectric RAM (FeRAM).

The time calculator 1128 may be configured to calculate local time or global time of the storage device 1100. The local time may be time elapsed in the storage device 1100. For example, while the storage device 1100 is supplied with power, the time calculator 1128 may be configured to calculate time based on an internal clock or a clock supplied from the host 1200.

The global time may be time elapsed in the user device 1000 including the storage device 1100. The time calculator 1128 may be configured to calculate the local time of the storage device 1100 and synchronize the local time with the time of the host 1200. The synchronized local time may be global time. For example, the global time may be real time.

The time calculator 1128 may be configured to calculate a reuse period of the flash memory 1110 using global time or local time. That is, the time calculator 128 may be configured to calculate a time interval between previous erase time and current erase time of the flash memory 1110 or a time interval between previous program time and current program time.

The time calculator 1128 may include registers, counters, and/or other circuits. The local time, global time, real time, or the like may be stored in and accessible in such registers, counters, and/or other circuits. The time calculator 1128 may include memory to store information such as time stamps, a number of erases a number of fast cycles, or the like.

FIG. 2 is a block diagram of an example of the flash memory 1110 in FIG. 1. As illustrated, the flash memory 1110 may include a memory cell array 1111, a data input/output (I/O) circuit 1112, an address decoder 1113, and a control logic 1114.

The memory cell array 1111 may include multiple memory blocks BLK1 to BLKz. Each of the memory blocks BLK1 to BLKz may have a two-dimensional structure or a three-dimensional structure. In a memory block having a two-dimensional structure, memory cells are formed in directions parallel to a substrate. Memory cells belonging to each memory block may be erased at the same time.

The data I/O circuit 1112 is connected to the memory cell array 1111 through multiple bitlines BLs. The data I/O circuit 1112 may be configured to receive data DATA1 from an external device or output the data DATA1 read from the memory cell array DATA1 to an external device. The address decoder 1113 is connected to the memory cell array 1111 through multiple wordlines WLs and selection lines GSL and SSL. The address decoder 1113 receives an address ADDR1 and selects a wordline.

The control logic 1114 may be configured to control operations such as program, read, and erase operations of the flash memory 1110. For example, during the program operation, the control logic 1114 may be configured to control the address decoder 1113 to provide a program voltage to a selected wordline and control the data I/O circuit 1112 to program data into memory cells connected to the selected wordline.

FIG. 3 is a circuit diagram of an example of the memory block BLK1 in FIG. 2. The memory block BLK1 has a three-dimensional structure in which memory cells are formed in a direction perpendicular to a substrate.

Referring to FIG. 3, cell strings CS11 to CS33 are coupled between bitlines BL1 to BL3 and a common source line CSL. Each of the cell strings (e.g., CS11) includes a ground selection transistor GST, multiple memory cells MC1 to MC8, and a string selection transistor SST.

The string selection transistor SST is connected to a corresponding string selection line SSL. In this embodiment, the string selection lines SSL include first to third string selection lines SSL1 to SSL3. The ground selection transistor GST is connected to a ground selection line GSL. Ground selection lines of each cell string are connected to each other. The string selection transistor SST is connected to a corresponding bitline BL, and a ground selection transistor GST is connected to a common source line CSL.

The memory cells MC1 to MC8 are connected to corresponding wordlines WL1 to WL8, respectively. A group of simultaneously programmed memory cells is called a page. The memory block BLK1 may include multiple pages. Multiple pages may be connected to a single wordline. Referring to FIG. 3, a wordline (e.g., WL4) of the same height from the common source line CSL is commonly connected to three pages.

FIG. 4 is a block diagram of an example of the memory controller 1120 in FIG. 1. As illustrated, the memory controller 1120 includes a bus 1121, a processor 1122, a RAM 1123, an ECC circuit 1124, a host interface 1125, a buffer control circuit 1126, and a memory interface 1127.

The bus 1121 may be configured to provide a channel between components of the memory controller 1120. The bus 1221 may be classified into a control bus and a data bus. The data bus may be configured to transmit data in the memory controller 1120, and the control bus may be configured to transmit control information such as a command and an address in the memory controller 1120. The data bus and the control bus may be separated from each other and may not interfere with each other or may have no influence on each other. However, in other embodiments, the control bus and the data bus may be combined, share components, or the like.

The processor 1122 may be configured to control the overall operation of the memory controller 1120 and perform a logical operation. The processor 1122 may be configured to communicate with the host 1200 through the host interface 1125. The processor 1122 may include a time calculator 1128. The time calculator 1128 may be software driven by the processor 1122 or hardware that is a part of a circuit of the processor 1122.

The processor 1122 may be configured to control the memory controller 1120 using codes. The processor 1122 may load codes from a nonvolatile memory (e.g., read only memory) in the memory controller 1120 or the flash memory 1110 (see FIG. 1).

The RAM 1123 may be configured to be used as a working memory, a cache memory or a buffer memory of the processor 222. The RAM 1123 may be configured to store codes and commands that the processor 222 executes or store data processed by the processor 1122. The RAM 1123 may include a static RAM (SRAM) or other types of RAM.

The ECC circuit 1124 may be configured to correct an error. The ECC 1124 may be configured to generate an error correction code (e.g., parity) for error correction based on first data DATA1 to be output to the memory interface 1127 or second data DATA2 received from the host interface 1125. The first data DATA1 and the parity may be provided to the flash memory 1110 through the memory interface 1127. The ECC circuit 1124 may be configured to correct an error of the received data DATA1 using the first data DATA1 and the parity received through the memory interface 1127. In some embodiments, the ECC circuit 1124 may be included in the memory interface 1127.

The host interface 1125 is configured to communicate with the host 1200 according to the control of the processor 1122. The host interface 1125 may be configured to receive a second command CMD2 and a second address ADDR2 from the host 1200 and exchange second data DATA2 with the host 1200.

The host interface 1125 may be configured to perform communication based on at least one of various communication protocols such as USB (Universal Serial Bus), SATA (Serial AT Attachment), SAS (Serial Attached SCSI), HSIC (High Speed Interchip), SCSI (Small Computer System Interface), Firewire, PCI (Peripheral Component Interconnection), PCIe (PCI express), NVMe (NonVolatile Memory express), UFS (Universal Flash Storage), SD (Secure Digital), MMC (MultiMedia Card), eMMC (embedded MMC), Fiber channel, Ethernet, remote direct memory access (RDMA), Infiniband, or the like.

The buffer control circuit 1126 may be configured to control the RAM 1123 according to the control of the processor 1122. The buffer control circuit 1126 may be configured to write data into the RAM 1123 and read data from the RAM 1123. When the RAM 1123 is disposed outside the memory controller 1120, the buffer control circuit 1126 may be configured to control the external RAM 1123.

The memory interface 1127 may be configured to communicate with the flash memory 1110 according to the control of the processor 1122. The memory interface 1127 may be configured to transmit a first command CMD1 and a first address ADDR1 to the flash memory 1110 and exchange the first data DATA1 and a control signal CTRL with the flash memory 1110.

FIG. 5 is a flowchart summarizing an example of operation of the time calculator 1128 in FIG. 4. The time calculator 1128 may calculate a reuse period of a selected memory block of the flash memory 1110 according to the control of the processor 1122.

In S110, the memory controller 1120 may read an erase time stamp ETS of a selected memory block. The erase time stamp ETS may be calculated by the time calculator 1128 and include a previous erase time of the selected memory block. The erase time stamp ETS may be stored in spare memory cells of the selected memory block or meta memory cells of a meta memory block and may be loaded in the RAM 1123.

In S120, the time calculator 1128 may calculate a reuse period using the erase time stamp ETS and a current time. The reuse period may be calculated through a time interval between current erase time and previous erase time read through the erase time stamp ETS.

At S130, the time calculator 1128 may recognize the current time as a new erase time stamp ETS of the selected memory block and provide the current time to the memory controller 1120. The memory controller 1120 may store the current time, as a new erase time stamp ETS of the selected memory block, in the spare memory cells of the selected memory block, the meta memory cells of the meta memory block or the RAM 1123.

When memory cells of the memory block BLK1 in FIG. 3 are programmed, charges may be trapped in insulating layers of the memory cells or insulating layers around the memory cells. When a predetermined time passes after the memory cells are erased and programmed, the charges trapped in insulating layers may return to the original state.

However, when the memory cells are re-erased and re-programmed within the predetermined time after they are programmed, the trapped charges may not return to the original state and may accumulate in the insulating layers. When the trapped charges are accumulated, the memory cells may be degraded. The degradation of the memory cells may cause data reliability of the storage device 100 to be degraded and may cause the life of the storage device 1100 to be reduced.

The storage device 1100 in FIG. 1 may manage the number of fast cycles of a selected memory block using a reuse period to reduce degradation of memory cells. In addition, the storage device 1100 may reduce the degradation of the memory cells to improve the data reliability of the storage device 1100 and increase the lifetime of the storage device 1100.

Hereinafter, time taken from erasing (E) and programming (P) a selected memory block to re-erasing (E) and re-programming (P) the memory block will be referred to as “a reuse period”. The reuse period may be defined as time taken from erasing a memory block to re-erasing the memory block or time taken from programming a memory block to re-programming the memory block.

FIG. 6 is a flowchart summarizing an example of an operation where the user device in FIG. 1 selects a memory block to be erased. Referring to FIG. 6, the memory controller 1120 counts the number of erases (NOE) of each memory block of the flash memory 1110 (S210). The memory controller 1120 counts the number of fast cycles (NOF) of each memory block of the flash memory 1110 (S220). The memory controller 1120 selects a memory block to be selected, based on the NOE and the NOF of each memory block of the memory controller 1120 (S230).

In some embodiments, the memory block selected to be erased may be the memory block with the least number of fast cycles NOF. In particular, the selected memory block may be a memory block with zero fast cycles NOF. In some embodiments, the memory block to be selected may be the memory block with the least number of erases NOE. In other embodiments, selecting the memory block may use a combination of the number of fast cycles NOF and the number of erases NOE of the blocks. For example, the selected memory block may be the memory block with the least fast cycles NOF and the least erases NOE.

The fast cycle occurs when each memory block is re-erased and re-programmed within a critical time after being erased and programmed That is, the fast cycle may occur when a reuse period of a selected memory block is shorter than the critical time. The NOF refers to the total execution number of fast cycles when an erase operation and a program operation are repeatedly performed on a selected memory block.

FIG. 7 is a conceptual diagram illustrating an example of the number of erases (NOE) and the number of fast cycle (NOF) of the memory block BLK1 described in FIG. 6. In FIG. 7, an erase operation (E) and a program operation (P) are repeatedly performed over time.

Referring to FIG. 7, after first time T1 has passed since an erase operation (E) and a program operation (P) were performed on the first memory block BLK1, the erase operation (E) and the program operation (P) are performed again. The first time T1 is a reuse period of the first memory block BLK1. The reuse period of the first memory block BLK1 is longer than critical time TCR that is the reference of a fast cycle. That is, after the first time T1, which is longer than the critical time TCR, has passed since the erase operation (E) and the program operation (P) are performed on the first memory block BLK1, the erase operation (E) is performed again. Thus, a fast cycle does not occur in the first memory block BLK1.

Alternatively, an erase operation (E) and a program operation (P) are repeatedly performed on a second memory block BLK2 during a reuse period that is second time T2 shorter than or equal to the critical time TCR. Each time the erase operation (E) and the program operation (P) are performed during the reuse period of the second time T2, a fast cycle occurs in the second memory block BLK2. Each time the fast cycle occurs, the number of fast cycles (NOF) increases. In the example of FIG. 7, the NOE of the first memory block BLK1 is 1 and the NOF is 0. The NOE of the second memory block is 3 and the NOF is 3. Although examples, where only fast cycles or no fast cycles have been performed on a memory block, in other embodiments, both fast cycles and slower cycles may be performed on a particular memory block.

FIG. 8 is a block diagram illustrating an example of an occurrence of fast cycles (FC) in FIG. 7. In an upper portion of FIG. 8, first to eighth memory blocks BLK1 to BLK8 are shown. In a lower portion of FIG. 8, a table shows the number of erases (NOE) and the number of fast cycles (NOF) of the first to eighth memory blocks BLK1 to BLK8.

Cold data CD may be programmed into the first to sixth memory blocks BLK1 to BLK6. The cold data CD is contrary to hot data HD and refers to data whose update frequency is relatively low. The cold data CD stored in the first to sixth memory blocks BLK1 to BLK6 may be data to be retained that are not frequently updated. That is, the NOE of the first to sixth memory blocks BLK1 to BLK6 is maintained without increase and the NOF thereof may be maintained at a state of zero.

The hot data HD may be programmed into seventh and eighth memory blocks BLK7 and BLK8 while the cold data CD is programmed into the first to sixth memory blocks BLK1 to BLK6. The hot data HD may be erased and programmed in the seventh and eighth memory blocks BLK7 and BLK8 during a short reuse period. Thus, NOE and NOF may increase in the seventh and eighth memory blocks BLK7 and BLK8.

FIG. 9 is a flowchart summarizing an example of a method for counting the number of fast cycles (NOF) in FIG. 7. Referring to FIG. 9, the memory controller 1120 erases a selected memory block (S310). The memory controller 1120 calculates a reuse period R/P using current erase time and previous erase time of a selected memory block (S320). The reuse period R/P of the selected memory block may be calculated by the time calculator 1128.

The memory controller 1120 determines whether the calculated reuse period R/P is smaller than or equal to a first critical value CR1 (S330). The first critical value CR1 may be critical time TCR dividing a fast cycle from other cycles. When the recycle period R/P is smaller than or equal to the first critical value CR1 (Yes), the flow proceeds to S340. The memory controller 1120 increases the NOF of the selected memory block (S340).

However, when the R/P of is greater than the first critical value CR1 (No), the flow proceeds to S350. The memory controller 1120 may manage the NOF of the selected memory block according to the method described in FIG. 10. The memory controller 1120 increases the NOE of the selected memory block (S360).

FIG. 10A is a flowchart summarizing an example of S350 in FIG. 9. FIG. 10 illustrates a method for the number of fast cycles (NOF) when a reuse period R/P between current erase time and previous erase time of a selected memory block is greater than the first critical value CR1 in FIG. 9.

At S410, the memory controller 1120 determines whether NOF of a selected memory block is greater than zero. When the NOF of the selected memory block is not greater than zero (No), the NOF of the selected memory block is not managed separately. When the NOF of the selected memory block is greater than zero, the flow proceeds to S420.

At S420, the memory controller 1120 determines whether a reuse period of the selected memory block is greater than a second critical value CR2. The second critical value CR2 may be greater than the first critical value CR1 in FIG. 9 and, in some embodiment, may be much greater than the first critical value CR1 (CR2>>CR1). The second critical value CR2 may be sufficient time required to return charges trapped in insulating layers of memory cells MC or insulating layers around the memory cells MC to the original state. The second critical value CR2 may be a predetermined value or a value varying depending on the NOF of the selected memory block. At S420, when the reuse period R/P is greater than the second critical value CR2 (Yes), the flow proceeds to S430.

At S430, the memory controller 1120 resets the NOF of the selected memory block. That is, the NOF of the selected memory block is initialized to zero. Then the management of the NOF of the selected memory block is completed. At S420, when the reuse period R/P is not greater than the second critical value CR2 (No), the flow proceeds to S440.

At S440, the memory controller 1120 determines whether the reuse period R/P of the selected memory block is greater than a third critical value CR3. The third critical value CR3 may be greater than the first critical value CR1 and smaller than the second critical value CR2 (CR1<CR3<CR2). The third critical value CR3 may be a time required to partially recover the charges trapped to the insulating layers of the memory cells MC or the insulating layers around the memory cells MC. At S440, when the reuse period R/P is greater than the third critical value CR3, the flow proceeds to S450.

At S450, the memory controller 1120 decreases the NOF. Then the management of the NOF of the selected memory block may be completed. At S440, even when the reuse period R/P is not greater than the third critical value CR3 (No), the management of the NOF of the selected memory block may also be completed.

FIG. 10B is a flowchart summarizing another example of S350 in FIG. 9. The operations of FIG. 10B may be similar to those of FIG. 10A. However, in this embodiment, operations S440 and S450 are not performed. As a result, the NOF of a selected memory block is reset in S430 only when the reuse period R/P of the selected memory block is greater than the second critical value CR2.

When a reuse period is short, data reliability of the flash memory 1110 in FIG. 1 may be degraded. The data reliability of the flash memory 110 may vary at each wordline WL in a single memory block. When a memory block is reused in a short period, data reliability of the overall memory block may be affected due to a wordline WL having the lowest reliability.

The storage device 1100 according to some embodiments may improve data reliability by not writing user data into a low-reliability wordline when a memory block is reused within a fast cycle period. When a reuse period of the memory block is fast, the storage device 1100 may dynamically adjust the number of wordlines that do not write user data according to the reuse period.

According to some embodiments, data reliability may be improved by not writing user data into a low-reliability wordline. In addition, the number of wordlines that does not write user data may be dynamically adjusted depending on the reuse period to reduce performance degradation of the storage device 1100.

FIG. 11 is a flowchart summarizing an example of a method in which the storage device in FIG. 1 dynamically controls wordlines of a memory block depending on a reuse period.

Referring to FIG. 11, at S510, the memory controller 1120 erases a selected memory block. At S520, the memory controller 1120 may calculate a reuse period R/P of the selected memory block using current erase time CET and previous erase time PET of the selected memory block. The reuse period R/P of the selected memory block may be calculated by the time calculator 1128.

At S530, the memory controller 1120 decides wordlines for writing the user data based on the reuse period. The memory controller 1120 may determine which wordline in the memory block is written with the user data, based on the reuse period.

At S540, the memory controller 1120 writes the user data into a wordline selected to write the user data. In a memory block, the memory controller 1120 may write dummy data into a wordline into which the user data is not written. Hereinafter, an operation of writing dummy data into a wordline into which user data is not written will be referred to as “dummy skip”.

FIG. 12 illustrates an example of an operation of determining wordlines for writing user data depending on the reuse period described in FIG. 11.

Referring to FIG. 12, the memory controller 1120 may vary the set of wordlines for writing user data depending on a reuse period R/P. When the reuse period R/P is longer than first critical time TH1 (R/P>TH1), the user data may be written into all wordlines WL1 to WL8. In this case, dummy data is not written into a wordline in a memory block (no dummy skip).

When the reuse period R/P is shorter than the first critical time TH1 (R/P<TH1), first dummy skip may be applied. In the first dummy skip, the user data may be written into the first to seventh wordlines WL1 to WL7 and the dummy data is written into the eighth wordline WL8 having the lowest reliability.

When the reuse period R/P is shorter than second critical time TH2 (R/P<TH2<TH1), second dummy skip may be applied. In the second dummy skip, the user data may be written into the second to seventh wordlines WL2 to WL7 and the dummy data may be written into the first and eighth wordlines WL1 and WL8 having the lowest reliability. When dummy skip is executed on multiple wordlines, the memory controller 1120 may sequentially select from a lower-reliability wordline.

In the second dummy skip, the user data is written into six wordlines WL2 to WL7 among the eight wordlines WL1 to WL8 of the memory block and the dummy data is written into two wordlines WL1 and WL8 having low reliability. According to some embodiments, since the user data is not written into the two wordlines WL1 and WL8 having low reliability, reliability degradation caused by fast reuse may be reduced. Although dummy data written to a single first wordline and/or a single last wordline are used as examples, in other embodiments, dummy data may be written to additional wordlines. For example, in some embodiments, dummy data may be written to wordlines WL1, WL2, WL7 and WL8.

In addition, since the number of wordlines into which user data is not written is dynamically adjusted, performance degradation caused by fast reuse may also be reduced. When a determination is made as to what number of wordlines are dummy-skipped, the number of the dummy-skipped wordlines may be decided as the number of wordlines that are not degraded in performance at the host interface 1125 (see FIG. 4). For example, when the host interface 1125 operates at about 500 MB/s and the flash memory 1110 operates at about 600 MB/s, five sixths of all the wordlines may be dummy-skipped.

FIG. 13 is a flowchart summarizing an example of a method for determining which one of a current reuse period (current R/P) and a previous reuse period (previous R/P) is used when dummy skip is executed depending on the reuse period described in FIG. 11. The selection of the reuse period may be carried out between S520 and S530 in FIG. 11.

Referring to FIG. 13, the memory controller 1120 compares a current R/P with a previous R/P (S521). The memory controller 1120 determines whether the current R/P is greater than the previous R/P (S522). When the current R/P is greater than the previous R/P (Yes), the previous R/P may be used (S523). Alternatively, when the current R/P is not greater than the previous R/P (No), the current R/P may be used (S524).

The memory controller 1120 may decide a wordline for user data depending on the current R/P or the previous R/P (S530). For example, assuming that the current R/P is 20 seconds and the previous R/P is 10 seconds (current R/P>previous R/P), the memory controller 1120 may determine which wordline is dummy-skipped on the basis of 10 seconds that is the previous R/P because the current R/P is greater than the previous R/P.

FIG. 14 is a flowchart summarizing an example of a method in which the storage device in FIG. 1 adjusts a voltage provided to a low-reliability wordline depending on a reuse period.

Referring to FIG. 14, at S610, the memory controller 1120 erases a selected memory block. At S620, the memory controller 1120 calculates a reuse period R/P of the selected memory block using current erase time CET and previous erase time PET of the selected memory block. The reuse period of the selected memory block may be calculated by the time calculator 1128.

At S630, the memory controller 1120 determines wordlines for normal programming based on the reuse period R/P. The memory controller 1120 may determine which wordline in the memory block is normally programmed depending on the reuse period R/P.

At S640, the memory controller 1120 provides a normal program voltage to a wordline determined to be normally programmed. Alternatively, the memory controller 1120 may perform a slow program operation on a wordline on which a normal program operation is not performed in a memory block. Hereinafter, an operation of performing a slow program operation on a wordline on which a normal program operation is not performed will be referred to as “slow skip”.

FIG. 15 illustrates an example of an operation of determining wordlines for a normal program operation depending on the reuse period described in FIG. 14.

Referring to FIG. 15, the memory controller 1120 may determine that some wordlines will be programmed using a normal program operation and other wordlines will be programmed using a slow program operation depending on a reuse period R/P. When the reuse period R/P is longer than first critical time TH1 (R/P>TH1), a normal program operation may be performed on all the wordlines WL1 to WL8. In this case, a slow program operation is not performed on a wordline in a memory block (no slow skip).

When the reuse period R/P is shorter than the first critical time TH1 (R/P<TH1), a first slow skip may be applied. In the first slow skip, a normal program operation may be performed on the first to seventh wordlines WL1 to WL7 and a slow program operation may be performed on the eighth wordline WL8 having the lowest reliability.

When the reuse period R/P is shorter than second critical time TH2 (R/P<TH2<TH1), a second slow skip may be applied. In the second slow skip, the normal program operation is performed on the second to seventh wordlines WL2 to WL7 and the slow program operation may be performed on the first and eighth wordlines WL1 and WL8, which may have a lower reliability. When the slow skip is applied to multiple wordlines, the memory controller 1120 may sequentially select from a lower-reliability wordline. Although slow programming used to program a single first wordline and/or a single last wordline are used as examples, in other embodiments, slow programming may be used to program additional wordlines. For example, in some embodiments, slow programming may be used to program wordlines WL1, WL2, WL7 and WL8.

FIG. 16 is a timing diagram illustrating an example of a normal program voltage for a normal program operation and a slow program voltage for a slow program operation in FIG. 15. The flash memory 1110 may sequentially increase a program voltage during a program operation as program loops are executed.

Referring to FIG. 16, a voltage Vs is used as a start voltage during the normal program operation. As the program loop is executed, a program voltage increases by ΔV. In contrast, a voltage Vs' that is lower than Vs may be used as a start voltage during the slow program operation. In addition, the program voltage may increase by ΔV that is smaller than ΔV′ during the slow program operation.

According to the method of operating the storage device 1100 in FIG. 14, since a program voltage for a lower-reliability wordline is lower, a threshold voltage distribution may be made denser and reliability degradation caused by fast reuse may be reduced. In addition, since the number of wordlines on which a slow program operation is performed is dynamically adjusted depending on the reuse period, performance degradation caused by fast reuse may also be reduced.

The storage device 1100 may reduce performance degradation resulting from a user pattern. For example, a write operation is repeatedly performed on the same address (i.e., hot spot write). A reuse period of an area having a size of 1 GB may be about three to about five times shorter than that of an area having a size of 3 GB or 5 GB. According to some embodiments, since a slow program operation is dynamically performed depending on a reuse period, performance degradation may be efficiently reduced to fit a memory size.

A storage device according to some embodiments may be used in various products. The storage device according to some embodiments may be implemented not only with electronic devices such as a personal computer, a digital computer, a camcorder, a handheld phone, an MP3 player, a PMP, a PSP, and a PDA but also with a memory card, a USB memory, a solid state drive (SSD), and the like.

FIG. 17 illustrates applying a storage device according to some embodiments to a memory card. As illustrated, a memory card system 3000 may include a host 3100 and a memory card 3200. The host 3100 may include a host controller 3110 and a host connection unit 3120. The memory card 3200 may include a card connection unit 3210, a card controller 3220, and a flash memory 3230.

The host 3100 may be configured to write data into the memory card 3200 or read data stored in the memory card 3200. The host controller 3110 may be configured to transmit a command (e.g., write command), a clock signal CLK generated by a clock generator (not shown) in the host 3100, and data DATA to the memory card 3200 through the host connection unit 3120.

The card controller 3220 may be configured to store data in the flash memory 3230 in synchronization with a clock signal generated by a clock generator (not shown) in the card controller 3220 in response to a write command received through the card connection unit 3210. The flash memory 3230 may be configured to store data transmitted from the host 3100. For example, when the host 3100 is a digital camera, the flash memory 3230 may store image data. The card controller 3220 may be configured to operate the flash memory 3230 as described above in various embodiments.

FIG. 18 is a block diagram illustrating applying a storage device according to some embodiments to a solid state drive (SSD). As illustrated, an SSD system 4000 may include a host 4100 and an SSD 4200.

The SSD 4200 may be configured to exchange a signal with the host 4100 through a signal connector 4211 and receive power through a power connector 4221. The SSD 4200 may include multiple flash memories 4201 to 420 n, an SSD controller 4210, and an auxiliary power supply 4220.

The flash memories 4201 to 420 n may be configured to be used as a storage medium of the SSD 4200. The SSD 4200 may be configured to use a flash memory, but a nonvolatile memory device such as PRAM, MRAM, ReRAM and FRAM may be used as memory in the SSD 4200. The flash memories 4201-420 n may be connected to the SSD controller 4210 through multiple channels CH1-CHn. One or more flash memories may be connected to one channel. Flash memory that may be connected to one channel may be connected to the same data bus. One or more flash memories may be connected to a single channel. Flash memories connected to the single channel may be connected to the same data bus.

The SSD controller 4210 may be configured to transfer/receive a signal SIG to/from the host 4100 through the signal connector 4211. The signal SIG may include, for example, a command, an address and data. The SSD controller 4210 may be configured to write data into a flash memory or read data from the flash memory. The internal configuration of the SSD controller 4210 will be described below in detail with reference to FIG. 19.

The auxiliary power supply 4220 may be connected to the host 4100 through the power connector 4221. The auxiliary power supply 4220 may be configured to be charged by receiving power PWR from the host 4100. The auxiliary power supply 4220 may be disposed inside or outside the SSD 4200. For example, the auxiliary power supply 4220 may be disposed on a mainboard and supply auxiliary power to the SSD 4200.

FIG. 19 is a block diagram illustrating an example of the configuration of the SSD controller 4210 in FIG. 18. As illustrated, the SSD controller 4210 may include a nonvolatile memory interface (NVM interface) 4211, a host interface 4:212, an error correction code (FCC) circuit 4213, a central processing unit (CPU) 4214, and a buffer memory 4215. In this embodiment, the signal connector 4211 will be referred to as the NVM interface 4211.

The NVM interface 4211 may be configured to distribute data transmitted from the buffer memory 4215 to channels CH1 to CHn. The NVM interface 4211 may be configured to transmit data read from flash memories 4201 to 420 n to the buffer memory 4215. The NVM interface 4211 may be configured to interface with flash memory. That is, the SSD controller 4210 may perform a program, read, and/or erase operation according to the manner of interfacing with flash memory.

The host interface 4212 may be configured to provide an interface with the SSD 4200 according to the protocol of the host 4212. The host interface 4212 may be configured to communicate with the host 4100 through interfaces such as Universal Serial Bus (USB), Small Component Small Interface (SCSI), Peripheral Component Interconnection (PCI) express, Advanced Technology Attachment (ATA), Parallel-ATA (RATA), Serial-ATA (SATA) and/or Serial Attached SCSI (SAS). The host interface 4212 may be configured to perform a disk emulation function in order for host 4100 to recognize the SSD 4200 as a hard disk drive (HDD).

The ECC circuit 4213 may be configured to generate an error correction code (FCC) using the data transmitted to the flash memories 4201 to 420 n. The generated ECC may be stored in a spare area of the flash memories 4201 to 420 n. The ECC circuit 4213 may be configured to detect an error of data read from the flash memories 4:201 to 420 n. If the detected error is a correctable error, the ECC circuit 4213 may be configured to correct the detected error.

The CPU 4214 may be configured to analyze and process a signal SGL input from a host 4100 (see FIG. 19). The CPU 4214 may be configured to control the host 4100 or the flash memories 4201 to 420 n through the host interface 4212 or the NVM interface 4211. The CPU 4214 may be configured to control the operation of the flash memories 4201 to 420 n according to firmware for driving the SSD 4200.

The buffer memory 4:215 may be configured to temporarily store write data provided from the host 4100 or data read from the flash memory. The buffer memory 4215 may be configured to store metadata or cache data to be stored in the flash memories 4201 to 420 n. During a sudden loss of power, the metadata or the cache data stored in the buffer memory 4215 may be stored in the flash memories 4201 to 420 n. A DRAM or an SRAM may be included in the buffer memory 4215.

FIG. 20 is a block diagram illustrating a storage device 5100 according to some embodiments is implemented with an electronic device. The storage device 5100 may be implemented in any electronic device 5000 such as a personal computer (PC) or a handheld electronic device such as a laptop computer, a mobile phone, a personal digital assistant (PDA), and a digital camera. The electronic device 5000 may include any electronic device that may use data storage.

As illustrated, the electronic device 5000 may include a storage device 5100, a power supply 5200, an auxiliary power supply 5250, a central processing unit (CPU) 5300, a random access memory (RAM), and a user interface 5500. The storage device 5100 may include a flash memory 5110 and a memory controller 5120.

As described above, memory blocks may be managed depending on a reuse period of a memory block. Thus, degradation of the memory block may be reduced and reliability of the memory block may be improved.

Some embodiments include a storage device with improved reliability and a method of operating the same.

Some embodiments include a method of operating a storage device including calculating a reuse period of a selected memory block in the flash memory, deciding a wordline for writing user data based on the reuse period of the selected memory block, and writing the user data into the decided wordline. Dummy data may be written into a wordline into which the user data is not written.

In some embodiments, the reuse period of the selected memory block may be calculated as a time interval between current erase time and previous erase time. When the reuse period of the selected memory block is longer than critical time, the dummy data may not be written into the selected memory block. When the reuse period of the selected memory block is shorter than the critical time, the user data may be written into a wordline decided to write the user data and the dummy data may be written into an undecided wordline.

Some embodiments include a method of operating a storage device including calculating a reuse period of a selected memory block in the flash memory, deciding a wordline for performing a normal program operation based on the reuse period of the selected memory block, and providing a normal program voltage to the decided wordline. The method may further include perform a slow program operation on a wordline on which the normal program operation is not performed. The slow program operation may use a lower program voltage than the normal program operation.

In some embodiments, when the reuse period of the selected memory block is longer than critical time, a slow program voltage may not be used in the selected memory block. When the reuse period of the selected memory block is shorter than the critical time, a normal program voltage may be provided to a wordline decided to perform a normal program operation and a slow program voltage may be provided to an undecided wordline.

The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other features, which fall within the true spirit and scope. Thus, to the maximum extent allowed by law, the scope is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description. While particular embodiments have been shown and described, it will be understood by one of ordinary skill in the art that variations in form and detail may be made therein without departing from the spirit and scope of the claims. 

What is claimed is:
 1. A method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining a set of wordlines of the selected memory block for writing data based on the reuse period of the selected memory block; and writing the data into the set of wordlines.
 2. The method as set forth in claim 1, further comprising writing dummy data into a wordline of the selected memory block not in the set of wordlines.
 3. The method as set forth in claim 2, wherein writing the data comprises: writing dummy data to a dummy wordline having a lower reliability of the selected memory block; and writing the data to wordlines of the selected memory block sequentially from the dummy wordline.
 4. The method as set forth in claim 1, wherein: the reuse period is referred to as a current reuse period; and the method further comprises comparing the current reuse period with a previous reuse period; and the determining of the set of wordlines is based on the comparison of the current reuse period and the previous reuse period.
 5. The method as set forth in claim 4, wherein the determining of the set of wordlines is based on the previous reuse period when the current reuse period is longer than the previous reuse period.
 6. The method as set forth in claim 1, wherein the calculating of the reuse period of the selected memory block comprises calculating a time interval between current erase time and previous erase time.
 7. The method as set forth in claim 1, wherein the calculating of the reuse period of the selected memory block comprises calculating a time interval between current program time and previous program time.
 8. The method as set forth in claim 1, wherein the determining of the set of wordlines comprises determining all of the wordlines of the selected memory block as the set of wordlines when the reuse period of the selected memory block is longer than a first threshold.
 9. The method as set forth in claim 8, wherein the determining of the set of wordlines comprises determining all of the wordlines except a first wordline of the selected memory block as the set of wordlines when the reuse period of the selected memory block is shorter than the first threshold.
 10. The method as set forth in claim 8, wherein the determining of the set of wordlines comprises determining all of the wordlines except a first wordline and a last wordline of the selected memory block as the set of wordlines when the reuse period of the selected memory block is shorter than a second threshold that is less than the first threshold.
 11. A method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining types of programming to use to program wordlines of the selected memory block based on the reuse period; and programming the wordlines using the types of programming.
 12. The method as set forth in claim 11, wherein the types of programming comprise a slow program operation and a normal program operation when the reuse period is less than a first threshold.
 13. The method as set forth in claim 12, wherein the slow program operation uses a lower program voltage than the normal program operation.
 14. The method as set forth in claim 12, wherein the types of programming comprise only the normal program operation when the reuse period is greater than the first threshold.
 15. The method as set forth in claim 12, wherein when the reuse period is less than the first threshold, the programming of the wordlines comprises: programming a first wordline of the wordlines using the slow program operation; and programming at least one of a remainder of the wordlines using the normal program operation.
 16. The method as set forth in claim 15, wherein when the reuse period is less than a second threshold that is less than the first threshold, the programming of the wordlines further comprises programming a last wordline of the wordlines using the slow program operation.
 17. A method of operating a storage device including a flash memory, comprising: calculating a reuse period of a selected memory block in the flash memory; determining a programming technique to use to program the selected memory block based on the reuse period; and programming the selected memory block using the programming technique.
 18. The method of claim 17, wherein determining the programming technique comprises: determining programming all wordlines of the selected memory block as the programming technique when the reuse period is in a first state; and determining programming less than all of the wordlines of the selected memory block as the programming technique when the reuse period is in a second state different from the first state.
 19. The method of claim 17, wherein determining the programming technique comprises: determining programming all wordlines of the selected memory block with a normal program operation as the programming technique when the reuse period is in a first state; and determining programming some of the wordlines of the selected memory block with a slow program operation as the programming technique when the reuse period is in a second state different from the first state.
 20. The method of claim 17, further comprising: recalculating the reuse period of the selected memory block; and changing the programming technique based on the recalculated reuse period. 